1. Field of the Invention
The present invention is related to valve position monitors for fluid distribution and process control systems. More particularly, the present invention is directed to a method and apparatus that provides an alarm when the position of a valve or movable member is changed.
2. Discussion of Related Art
Electrical signaling systems for detecting a change in the position of a valve actuator are well known. For example, U.S. Pat. No. 4,721,131 discloses a valve security system having optical or electrical sensor devices installed on the axially-movable stem of each valve. This sensor device includes an adjustable lever which permits the device to be adapted for use with different valve stem diameters and valve stems having different thread pitches. However, this device must be mounted on the valve stem, which means it cannot be used if the valve stem does not have sufficient length to accommodate the sensor device. Furthermore, this is a complex device requiring precisely-formed mechanical parts, which increase the cost of the device.
U.S. Pat. No. 4,234,879 discloses a simple device suitable for monitoring handwheel-type valves regardless of the length of the valve stem. With this device, however, the movement of the handwheel causes the device to break an electrical circuit by disconnecting conductors in the plug and socket elements of the device. This is unacceptable in many applications, in that breaking the circuit between these conductors involves risks of spark and shock, and possibly also the shorting of the plug prongs to ground. In hospital, chemical manufacturing and laboratory environments, such a risk of sparks and shorting is unacceptable.